This invention relates to a printhead and a printing apparatus using the printhead, and more particularly, to an ink-jet printhead with a malfunction prevention function and a printing apparatus using the printhead.
In a conventional printhead to perform printing in accordance with an ink-jet method, an electrothermal transducer (heater) and a driver for the transducers are formed on the same substrate by using a semiconductor process technique, as disclosed in Japanese Patent Publication Laid Open No. 5-185594.
This printhead is mounted on a carriage of a printing apparatus, receives various control signals and an image signal from the printing apparatus, and operates based on the received signals. When the printhead is attached to the carriage, contacts provided in the printhead and contacts provided in the carriage are pressed into contact and electrical connection is established therebetween, then signals are supplied from the printing apparatus to the printhead.
To establish the electrical connection between the printhead and the printing apparatus, conventionally, needles for contacts are provided on the printing apparatus main body side on which the printhead is mounted, and the contacts of the printhead are pressed against the needles.
However, if a part of the electrical connection between the contacts is cut off due to some reason such as vibration of the apparatus or an error in attachment of the printhead to the carriage, heat generation by the heater not based on image data (i.e., erroneous ink discharge) and breakage of the heater due to the heat generation might occur.
Specifically, if an electrical connection is cut off by positional shift of the contacts of the printhead and the carriage or the like, there is a possibility that only the power supply voltage (VH) to the printhead is connected but the printhead cannot not be controlled. If such a trouble occurs, the power transistor might be continuously turned on, and at the worst, the heater might be heavily damaged.
Accordingly, it is an object of the present invention to provide a printhead and a printing apparatus using the printhead which protect the printhead without causing the printhead to malfunction even if an electrical connection between the printhead and the printing apparatus holding the printhead is accidentally cut off for some reason.
According to one aspect of the present invention, the foregoing object is attained by providing a printhead, supplied with a control signal, print data and electric power supplied via a plurality of electrical contacts, for performing printing operation, comprising: a printing element; a first contact that inputs a voltage to drive the printing element; a control circuit that performs drive control on the printing element; a second contact that inputs a voltage to drive the control circuit; a monitoring circuit that monitors the voltage in the second contact; and a protection circuit that forcefully stops the drive control on the printing element by the controller, in accordance with the result of monitoring by the monitoring circuit.
Preferably, the printing element includes a heater and a power transistor to electrify the heater, and the heater heats ink to cause film boiling in the ink and discharge the ink by a pressure of a bubble generated by the film boiling.
Further, it is preferable that the printhead further comprises: a third contact that inputs print data; and a memory circuit that is used for temporarily storing the print data inputted via the third contact. In this case, the control circuit inputs a signal representing the print data stored in the memory circuit and amplifies the voltage of the signal. The control circuit is a CMOS circuit comprising a PMOS transistor and an NMOS transistor.
Further, it may be arranged such that the protection circuit includes an analog switch and a PMOS transistor, and the analog switch cuts off an output from the control circuit in accordance with the result of monitoring from the monitoring circuit, on the other hand, the PMOS transistor forcefully turns off an output to the printing element; or it may be arranged such that the protection circuit includes a switch comprising a pair of a first PMOS transistor and an NMOS transistor and a second PMOS transistor, and the switch comprising the pair of the first PMOS transistor and the NMOS transistor cuts off an output from the control circuit in accordance with the result of monitoring from the monitoring circuit, on the other hand, the second PMOS transistor forcefully turns off an output to the printing element.
Further, it may be arranged such that the monitoring circuit operates by the voltage inputted from the first contact, and the monitoring circuit comprises a plurality of inverter circuits connected to the second contact and a pull-down resistor connected to the second contact; or it may be such that the monitoring circuit comprises a comparator having a first terminal connected to the second contact and a second terminal to input a voltage obtained by dividing the voltage inputted from the first contact by serially-connected first and second resistors, and a pull-down resistor connected to the second contact.
According to another aspect of the present invention, the foregoing object is attained by providing a printing apparatus using a printhead having the above construction.
In accordance with the present invention as described above, the voltage supplied to the control circuit to perform the drive control on the printing element via the second contact is monitored, and in accordance with the result of monitoring, the drive control on the printing elements by the control circuit is forcefully stopped.
The present invention is particularly advantageous since malfunction of the printing element caused by poor contact can be prevented.
For example, in a case where the printing element includes the heater, breakage of the heater and its driving circuit due to heat generation and overheating by malfunction can be prevented.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same name or similar parts throughout the figures thereof.